[bizerl (OP)]

It seems like a simple enough idea, but my (limited) understanding of the RGB spectrum was that it was the manifestation of different frequencies of electro-magnetic radiation.

In sound, you can add various pure-tone sine waves to make complex sound signals, but with light, you add two "pure-tone" sine waves of electro-magnetic radiation and you get...

ANOTHER PURE-TONE SINE WAVE OF ELECTRO-MAGNETIC RADIATION!! (i think).

Is this a trick of the light as it is picked up by our cones and/or rods?

[grizelda]

Probably has to do with the fact that sound travels through a medium, so additional frequencies re-modulate a medium which is already being modulated. Light doesn't interact with a medium, so additional frequencies are mathematically added to the original frequency. No change is made to the shape of the waveform because it doesn't have one.

[damocles]

It is a trick -- or rather a mode of reaction -- of the cones in our visual system.
The yellow that we observe from a sodium emission spectrum is quite different from the yellow that we perceive indistinguishably in transmission from a sodium chromate solution. The former comes from a wave that is the sum of just two wavelengths of yellow light (possibly made slightly orange by admixture of a weaker red emission line). The latter is a mixture of all wavelengths of green, yellow, and orange light that are the only visible light that is not absorbed by the solution as white light passes through it.

Irrelevant, but chemically it is possible to exactly match the subjective colours of the chromate solution and sodium lamp in a particular situation by lowering the pH to increase the amount of orange dichromate in the yellow chromate solution.

The cones in our eyes are tuned to respond to three separate peak wavelengths, with a sensitivity spread that looks roughly like a set of three overlapping gaussian peaks. The same set of three numbers given by the strength of the eye's response to the three key frequency ranges might therefore represent either a single frequency or a broad frequency spread. Of course, for some particular colours -- brown, pink, magenta -- the signal received by our eye could not correspond to any single frequency.

[CliffordK]

Keep in mind that there are two different ways to "mix colors".

When mixing paints, one may absorb more wavelengths of light, and reflect less.  So, mixing all paints together, and one ends up with black (all light absorbed, none reflected)
Mixing lights, on the other hand, one adds more and more wavelengths of light, and ends up with white.

The colors one gets from mixing paints and mixing lights are different.

The French Impressionists were experimenting with mixing light by combining the dots of pure colors.

I believe that color laser printers also print dots of pure color, whereas the color inkjets may mix the colors somewhat.  The colors on your computer screen are also made up of 3 colors, Red, Green, and Blue, of varying intensity.

As Damocles put it.  One has 3 cones responding to varying overlapping colors spectra.

Pure yellow would cause a probability of each cone being activated, which the eye/brain then interprets as yellow. 
If one uses mixed colors of multiple wavelengths that give the same exact probability that the three cones are activated, then the eye should interpret it as yellow.

[evan_au]

The three cones of human vision have a frequency response shown here: http://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception
The rainbow is made up of pure colours, which generally group into 6 or 7 names (depending on what you were taught when you started school).

The hairs of the cochlea respond to many different frequencies, and can distinguish the 88 notes on a piano keyboard (plus more) - although not many of us could name a specific note we heard! http://en.wikipedia.org/wiki/Cochlea#Summary

You could display a mixture of Red and Green colour, which will strongly stimulate the both the Medium and Long-wavelength sensitive cones. A pure Yellow frequency from the rainbow will also stimulate the same Medium and Long-wavelength sensitive cones, and so be perceived as the same colour.

But this does not imply that your eyes are performing a non-linear mixing of two light wavelengths to come up with a third intermediate wavelength. It is just that the two wavelengths stimulate the same cones at the same strength as a single, intermediate wavelength.

A counter-example to this theory is if you mix Red and Blue light, you end up with Magenta, which is not one of the pure colours between Red and Blue. See the colour circle at:
http://en.wikipedia.org/wiki/Primary_colour#Additive_primaries

[Monox D. I-Fly]

Pure yellow would cause a probability of each cone being activated, which the eye/brain then interprets as yellow. 
If one uses mixed colors of multiple wavelengths that give the same exact probability that the three cones are activated, then the eye should interpret it as yellow.

So is it why old shabby clothes which were once white tend to turn yellow instead of grey?

[chris]

Do you mean once you have washed them?

[chiralSPO]

So is it why old shabby clothes which were once white tend to turn yellow instead of grey?

Pigments appear yellow because they absorb strongly in the violet region of the spectrum (if you look at a color wheel you will see that these are opposite colors). Many things that are "supposed" to be colorless (white) actually have a slight yellowish or brownish tint because of compounds that absorb in the ultraviolet. If there is enough of these compounds then the tail end of that UV absorption can bleed into the visible region, wiping out the violet light (then into the blue etc.) This is responsible for the yellow colors of sulfur and butter, and the brownish tint of unbleached flour or paper.

Most natural materials are not the brilliant white that we think of as white. To achieve this effect we bleach the materials,
and add phosphors which absorb UV rays and then emit in the visible region (so the clothes appear brighter than white) or add very finely powdered titanium dioxide or zin oxide (this last approach is used for white paints and confectioner's sugar, among other products)

[hamdani yusuf]

In sound, you can add various pure-tone sine waves to make complex sound signals, but with light, you add two "pure-tone" sine waves of electro-magnetic radiation and you get...

ANOTHER PURE-TONE SINE WAVE OF ELECTRO-MAGNETIC RADIATION!! (i think).
..

Then you think wrong. The result is only ANOTHER PURE-TONE SINE WAVE OF ELECTRO-MAGNETIC RADIATION if the interfering waves have exactly the same frequency.

[Jeaniecary]

I agree with this.

[evan_au]

Donald Trump thinks that energy efficient lighting add to his skin tones to make him look orange...

See, for example: https://www.washingtonpost.com/politics/2019/09/13/trump-blamed-energy-saving-bulbs-making-him-look-orange-experts-say-probably-not/?noredirect=on

[Bored chemist]

Donald Trump thinks that energy efficient lighting add to his skin tones to make him look orange...

See, for example: https://www.washingtonpost.com/politics/2019/09/13/trump-blamed-energy-saving-bulbs-making-him-look-orange-experts-say-probably-not/?noredirect=on

It is particularly impressive that they can do this when he is outside in sunshine. Furthermore, they don't have this effect on other people who are stood next to him.

Incidentally, in belated reply to the OP's question; what else could mixing colours produce?

[evan_au]

what else could mixing colours produce?

14, A-flat, or a poke in the ribs (if you were a synesthete).

See: https://en.wikipedia.org/wiki/Synesthesia

[Bored chemist]

what else could mixing colours produce?

14, A-flat, or a poke in the ribs (if you were a synesthete).

See: https://en.wikipedia.org/wiki/Synesthesia

That would still be a colour- even if it smelled of mint.

[sciencefreak24]

Color perception is a function of the cone receptors in our eyes. There are three distinct ranges of frequencies they are sensitive to (centered on red, green and blue). The actual determination of a "color" comes from our brain, taking the three values measured by the cones and representing that in our visual perception as a single color for that clusters of receptors.

Digital cameras work in the same way, measuring RGB frequencies of light and recording that as a single "color". Computer monitors display them in the same way, using RGB pixels to mix the light together and make our eyes see yellow, orange or whatever.

You can't mix frequencies we can't see and get "colors". All color perception requires our receptors to measure the light. A lack of light is just black to our eyes.

[Bored chemist]

You can't mix frequencies we can't see and get "colors".

That's exactly what happens in green laser pointers.
Invisible  IR radiation is mixed (with itself) to give visible green light.

[evan_au]

Digital cameras work in the same way, measuring RGB frequencies of light and recording that as a single "color".

It's true that Digital cameras have 3 RGB color sensors (a single design of silicon sensor is covered by 3 different colored Red, Green & Blue filters to mimic human vision).

In "raw" mode, the RGB readings are saved separately, as they are measured. This can then be presented by the RGB dots on your computer monitor. It is only your eye that turns this mixture of RGB into a single color, to which you can attach a name.

If you can't afford the considerable memory capacity  needed for "raw" images, the processing is a little more complex:
- The image is compressed using a Discrete Cosine Transform (DCT)
- My understanding is that the DCT is applied separately to the RGB channels
- And the Inverse DCT is used to extract the RGB channels for display on a HDMI monitor (for example)

See: https://en.wikipedia.org/wiki/Modified_discrete_cosine_transform

If color palette mapping is used (eg in PNG pixel format 3), the most common RGB values in the image are detected, and the color of each pixel is matched to the nearest one of these. This mapping is reversed back into separate RGB values for display.
- This technique is effective for images with limited color choice (eg graphs and diagrams), but introduces distortions for images of the real world (for which PNG provides pixel format 2).
- The DCT is more effective for compressing the graded and blended of colors in the real world
https://en.wikipedia.org/wiki/Image_compression
https://en.wikipedia.org/wiki/Portable_Network_Graphics#Pixel_format